1. Field of the Invention
The present invention relates to systems providing an accurately variable known concentration of one gas in a mainstream of another gas, and is particularly concerned with such systems for use in calibrating instruments such as gas detection devices. The mainstream will usually be air and will, for convenience, be referred to hereafter as an airstream.
2. Discussion of the Prior Art
Gas detection devices which provide a measure of tracer or pollutant gas (henceforth, for convenience, referred to as test gas) in an airstream are well known. Devices which operate on the principle of ionisation of the test gas by means of ultra-violet light are described in, for example, GB 1576474 and WO 93/12354. In the devices described in these documents an airstream containing test gas is subjected to ultra-violet light and is passed between a pair of electrodes. Ionisation of the test gas results in the flow of electrical current between the electrodes and measurement of this current can, with calibration of the devices, provide an indication of the concentration of the test gas in the airstream.
The conventional method of calibration relies on commercially provided supplies of bottled gases containing requisite ratios of calibration gases in an undetectable carrier/buffer. This method has many disadvantages, amongst which are;
economy, in that successful calibration requires a large number of different mixtures, and in that each separate mixture is seldom completely used: in fact each calibration usually only requires only very small quantities of each mixture, and at relatively infrequent intervals,
practicality, in that detection devices of the type referred to require calibration on site, which requires a disproportionate effort to be devoted to the transport and manipulation of calibration equipment and mixtures, and
technical validity, in that the calibration gases are supplied as nominal mixtures, which require Further costly assay to confirm their precise composition. Furthermore the carrier gas itself may be contaminated. It is also preferable that the actual test gases which will be used or met in the field should be used in the calibrations rather than commercial equivalents.
U.S. Pat. No. 4,094,187 describes a stack gas analyzing system with a built in calibrating capability. Samples of stack gas are collected by a stack probe and passed to a series of gas analyzers, there being a separate analyzer for each possible contaminant gas. Sources, one for each suspected contaminant gas, provide supplies of contaminant gases which can be diluted by atmospheric air and fed to the gas analyzers either through the stack probe or directly. Concentrations of contaminant gases in the calibration flow are established by means of a flowmeter directly downstream of each gas supply and a flowmeter downstream of the air/contaminant gas mixing point. Delivery of calibration gas supplies must be carried out at pressures which cause displacement of stack gas from the analyzers.
The instruments described in GB1576474 and WO 93/12354 are field instruments which by reason of their function must be portable. This makes the inclusion of a built-in calibration means such as that described in U.S. Pat. No. 4,094,187 impracticable.